Screen, its manufacturing method and image display system

ABSTRACT

A projection screen comprises a red-reflecting particle layer, green-reflecting particle layer and blue-reflecting particle layer sequentially stacked on a substrate. In each particle layer particles are accumulated by eleven cycles in a regularly alignment such as close-packed structure. Diameter of red-reflecting particles is approximately 280 nm, diameter of green-reflecting particles is approximately 235 nm, and diameter of blue-reflecting particles is approximately 212 nm. Each particles layer is accumulated by self-organized technique. The substrate used here can absorb light of wavelengths other than those of red, green and blue three primary colors.

This application is a division of U.S. application Ser. No. 10/181,240,filed Feb. 5, 2003, now U.S. Pat. No. 6,961,175 which is a 371 ofPCT/JP02/02897, filed Mar. 26, 2002, which is hereby incorporated byreference in its entirety herein.

TECHNICAL FIELD

This invention relates to a screen, its manufacturing method and imagedisplay system especially suitable for use in projection of variouskinds of images.

BACKGROUND ART

Conventionally, projector screens are basically white background screensthat can reflect or scatter almost all part of light in the visiblewavelength region. In this case, since such a screen also scatters orreflects external light irrelevant to images if it intrudes onto thescreen, images deteriorate in contrast. Therefore, projection is usuallycarried out in a dark room.

However, even during projection in a dark room, any light filtering fromthe exterior or any light irrelevant to images inside the dark roomdeteriorates the contrast of the images, and raises the luminance levelof black-displaying portions.

In displays such as CRT or liquid crystal displays that are widely used,since light of each primary color has a wide spectral full width at halfmaximum (FWHM), the color reproduction range on a chromaticity diagramis narrow, and it is difficult to represent pure colors. Also in liquidcrystal or CRT projector-type displays, light forming an image scatteredor reflected from a screen similarly has a wide spectral full width athalf maximum, the color reproduction range on a chromaticity diagram isnarrow, and it is difficult to represent pure colors.

It is therefore an object of the invention is to provide a screen thatcan presents clean images in which black-displaying portions are loweredin luminance level even upon intrusion of any external light irrelevantto images, without deteriorating the contrast of images, and also toprovide a manufacturing method of the screen and an image display systemusing the screen.

A further object of the invention is to provide a screen notdeteriorating the contrast of images even under an ordinary fluorescentlamp or in the open air and therefore not requiring projection in a darkroom, and also to provide a manufacturing method of the screen and animage display system using the screen.

A still further object of the invention is to provide a screen that canmaintain a high contrast by efficiently, selectively reflecting light ofimages exclusively and cutting light of other wavelengths and cansimultaneously lower the luminance level of black-displaying portions inthe case where the full width at half maximum of emission spectrum of asemiconductor laser or a light emitting diode (LED) is narrow and imagesare formed by projecting light excellent in color purity, and to providea manufacturing method of the screen and an image display system usingthe screen.

A yet further object of the invention is to provide a screen that canensure a wide color reproduction range on a chromaticity diagram evenwhen projecting light from a display like a liquid crystal projector,for example,having a wide full width at half maximum of the spectrum ofeach primary color, and can represent pure colors, and to provide amanufacturing method of the screen and an image display system using thescreen.

DISCLOSURE OF INVENTION

To accomplish the above-indicated objects, according to the first aspectof the invention, there is provided a screen comprising a structure inwhich particles having a size not larger than 1 μm are regularlyaligned.

According to the second aspect of the invention, there is provided ascreen configured to reflect light of specific wavelengths by using aphotonic crystal.

The photonic crystal is an artificial crystal made by regularly aligningtransparent mediums (for example, two different kinds of transparentmediums) largely different in refractive index (dielectric constant) tocycles near the wavelength of light, e.g. to cycles of hundreds to onethousand and hundreds of nanometer. Depending on the order of theperiodical structure, it is called a one-dimensional photonic crystal,two-dimensional photonic crystal or three-dimensional photonic crystal.The photonic crystal is equivalent to the regularly aligned structure ofparticles in the feature having a periodical structure and having thefunction of reflect light. In other words, the regularly alignedstructure of particles may be regarded as one kind of photonic crystals.

According to the third aspect of the invention, there is provided ascreen comprising a structure using dielectric multi-layered films toreflect light of specific wavelengths.

The dielectric multi-layered film may be regarded as a one-dimensionalphotonic crystal.

According to the fourth aspect of the invention, there is provided amethod for manufacturing a screen having a structure in which particleshaving a size not larger than 1 μm are regularly aligned, characterizedin that the particles are aligned by self-organization.

Self-organization generally pertains to autonomously systemizing oneselfin accordance with an external information structure, but herein, itpertains to autonomous accumulation of particles and regular alignmentthereof in a system letting the particles accumulate (for example, aliquid) in accordance with parameters characterizing the system.

Accumulation of particles by self-organization is typically broughtabout in the following manner.

That is, according to the fifth aspect of the invention, there isprovided a method for manufacturing a screen having a structure in whichparticles having a size not larger than 1 μm are regularly aligned,comprising:

a first step of immersing a substrate into a particle solutioncontaining 2 weight % of particles;

a second step of wetting the surface of the substrate with the particlesolution by pulling up the substrate into air at a speed not slower than30 μm/s; and

a third step of drying the substrate wet with the particle solution inair.

Most preferably, the first to third steps are repeated until a desiredoptical property of the regularly aligned structure of the particles,i.e. the particle layer, is obtained, or until it reaches a desiredthickness. With one cycle of the first to third steps, it is difficultto obtain a uniform thickness of the particle layer on the plane of thesubstrate surface. Preferably, therefore, before immersing thesubstrate, during immersion (before pull-up) of the substrate orimmediately after pull-up of the substrate, the substrate is changed inorientation by rotating it within the plane of its own. In this case,thickness of the particle layer within the plane of the substrate may bechecked after drying the substrate so as to control the orientation ofthe substrate in accordance with the result. Concentration of theparticle solution as much as two weigh % is normally satisfactory tosmoothly proceed with accumulation of the particle layer. However, fromthe viewpoint of efficiently stacking the particle layer, a higherconcentration is desirable. On the other hand, although depending uponthe material of the particles, if the concentration is higher than 50weight %, it prevents good formation of the particle layer. Therefore,the concentration is preferably controlled not to exceed 50 weight %. Asto the pull-up speed of the substrate, a speed not slower than 30 μm/sis normally sufficient to proceed with deposition of the particle layerwithout problems. However, if the pull-up speed is excessively slow,thickness of the deposited particle layer tends to increase. Therefore,from the viewpoint of efficiently accumulating the particle layer, ahigher speed is desirable. The pull-up speed is considered to have noupper limit, but from the practical viewpoint, it is usually limited notto exceed 3 m/s.

According to the sixth aspect of the invention, there is provided animage display system comprising:

a screen configured to reflect light of specific wavelengths by usingphotonic crystals; and

a projector light source including semiconductor light emitting-devicesfor emitting light of the specific wavelengths.

According to the seventh aspect of the invention, there is provided animage display system comprising:

a screen having a structure in which particles having a size not largerthan 1 μm are regularly aligned; and

a projector light source including semiconductor light emitting deviceseach for emitting light of a specific wavelength determined by the sizeand alignment of the particles.

According to the eighth aspect of the invention, there is provided animage display system comprising:

a screen configured to reflect light of specific wavelengths by using adielectric multi-layered film; and

a projector light source including semiconductor light emitting devicesfor emitting light of the specific wavelengths.

In the present invention, the reason why the size of the particles usedfor the screen is limited not to exceed 1 μm lies in that, consideringthe substantially proportional relation between the size of theparticles and the wavelength of light reflected by the particles, thesize of the particles must be limited not to exceed 1 μm for reliablyreflect visible light contributing to formation of images. Especiallywhen the particles are aligned into a close-packed structure, toreliably reflect the light of three primary colors, the size of theparticles should be typically controlled in the range from 150 nm to 320nm approximately.

Basically, any method can be used to stack the particles used forforming the screen provided it can form a regularly aligned structure.Typically, however, the particles can be readily accumulated by usingself-organized technique. The particles are typically aligned into aclose-packed structure. The close-packed structure is either aclose-packed cubic structure in which the particles align to form aface-centered cubit lattice or a hexagonal close-packed structure inwhich the particles align to form a close-packed hexagonal lattice.

Typically for enabling simultaneous reflection of light of wavelengthscorresponding to red, green and blue three primary colors, a structureis employed, which includes three kinds of diameters of particles orthree kinds of cycles of the photonic crystals or dielectricmulti-layered films. Various granular materials are usable as theparticles, and any may be selected depending of its use. Preferably,silica particles or other particles having the same refractive index asthat of silica are used. Refractive index of silica is generally in therange of 1.36 to 1.47, although it may change depending on conditionsused for its fabrication. In this case, regardless of the material ofthe particles, when the refractive index of the particles is n, here areused particles having a diameter in the range from 269×(1.36/n) nm to314×(1.36/n) nm for reflecting red, particles having a diameter in therange of 224×(1.36/n) nm to 251×(1.36/n) nm for reflecting green, andparticles having a diameter in the range from 202×(1.36/n) to224×(1.36/n) nm for reflecting blue. More typically, here are usedparticles having a diameter in the range from 278×(1.36/n) nm to305×(1.36/n) nm for reflecting red, particles having a diameter in therange of 224×(1.36/n) nm to 237×(1.36/n) nm for reflecting green, andparticles having a diameter in the range from 208×(1.36/n) to217×(1.36/n) nm for reflecting blue. However, these red-reflectingparticles, green-reflecting particles and blue-reflecting particles maybe of different materials, if necessary. To enable simultaneousreflection of light of wavelengths corresponding to red, green and bluethree primary colors, photonic crystals or particle layers forreflecting red, green and blue, respectively, are stacked on asubstrate. The stacking order of these photonic crystals or particlelayers is basically free, but photonic crystals or particle layers forred reflection, green reflection and blue reflection may be stacked inthis order, or vice versa. The former stacking configuration isadvantageous for minimizing influences of Rayleigh scattering whereasthe latter is advantageous especially for improving the crystal propertyof the particle layers. In this case, the stacking period of photoniccrystals or particles layers for respective colors is preferably in therange from eight cycles to fifteen cycles for enhancing the wavelengthselectivity.

The photonic crystals or particle layers for red reflection, greenreflection and blue reflection. may be arranged in a lateral array onthe substrate. Here again, the stacking period of photonic crystals orparticles layers for respective colors is preferably in the range fromeight cycles to fifteen cycles for enhancing the wavelength selectivity.The particle layers for red reflection, green reflection and bluereflection may be stripe-shaped, rectangular-shaped or square-shaped,and they are arranged in a predetermined alignment pattern on thesubstrate. The order of alignment of these photonic crystal or particlelayers for red reflection, green reflection and blue reflection isbasically free.

In order to absorb visible light of wavelengths other than those of red,green and blue three primary colors, which pass through the photoniccrystals, particle layers or dielectric multi-layered films, the screenpreferably includes a layer of a bulk substrate capable of absorbingthose parts of visible light. Most preferably, this layer or bulksubstrate absorbs visible light of all wavelength bands. The layer orbulk substrate absorbing these parts of visible light preferablyunderlies the photonic crystals, particles or dielectric multi-layeredfilms (on the back surface viewed from the screen-watching direction). Atransparent substrate having a layer for absorbing visible light on theback surface thereof may be used as the substrate. Various materials areusable for forming the substrate, such as carbon and other inorganicmaterials, polyethylene terephthalate (PET) and other polymericmaterials, organic materials like resins, and complex materialscombining inorganic materials and organic materials. In case theparticle layers or photonic crystals are formed on the substrate in aliquid-phase, some kinds of substrates are not sufficiently wettable. Inthis case, preferably before the particle layers or photonic crystalsare formed, the substrate is treated to improve the wettability of itssurface. More specifically, an irregularity may be formed on thesubstrate surface by a surface-roughening technique, the surface may becoated with a SiO₂ film or the like may be coated, or the surface may beprocessed with a chemical liquid, for example. Furthermore, in case theparticle layers are stacked on the substrate in a liquid, a buffer layerof particles is preferably formed on the substrate beforehand to improveits wettability. Diameter of particles in the particle layer used as thebuffer layer is controlled to be smaller than the diameter of theblue-reflecting particles, which is in the range from 208×(1.36/n) to217×(1.36/n) nm. That is, it is controlled to be smaller than208×(1.36/n) nm. Although it depends on the substrate material, if thethickness of the substrate is controlled not to be smaller than 20 μm,then the substrate is advantageous, in general, for sufficient strengthas the screen that is unlikely to break. On the other hand, if thethickness is not larger than 500 μm, then the screen is more flexibleand convenient for handling when rolling it or transporting it. In casethe dielectric multi-layered films are used for making the screen, theperiodical structure of each dielectric multi-layered film preferablyincludes 10 cycles or more to enhance its wavelength selectivity.

To diverge the reflected light by using a diffraction effect, thelateral size of the photonic crystals or the aggregate of particles islimited not to exceed 22 cycles. Alternatively, it is possible to usephotonic crystals, an aggregate of particles or dielectric multi-layeredfilms that has, in combination, slanted surfaces and another surfacehaving a different angle from that of the slanted surfaces. In thiscase, the angle θ of the slanted surfaces is adjusted in the range from70°≦θ≦90°. Alternatively, the photonic crystals, aggregate of particlesor dielectric multi-layered films may have a curved surface. It is alsoacceptable to incline the crystal axis of the photonic crystals,aggregate of particles or dielectric multi-layered films by an angle ain the range from 77.4°≦θ≦90° from the incident direction of light.Further, from the viewpoint of moderating the directivity of lightreflected by the screen, the photonic crystals, aggregate of particlesor dielectric multi-layered films may have undulations. Furthermore,making an irregularity on the substrate surface also contributes tomoderate the directivity of light.

For the purpose of moderating the directivity of light reflected by thescreen and uniforming the luminosity over the entire screen, alight-diffusing medium is provided by coating or other appropriatetechnique. Specifically, the light-diffusing medium may be a diffusionfilm, micro lens film or micro prism film made, for example, which ismade of a polymeric material. To improve the mechanical strength of thescreen, gaps among the particles are buried with a binder of a polymericmaterial. In this case, the particles are changed to voids.

According to the ninth embodiment of the invention, there is provided ascreen comprising particles regularly aligned to reflect electromagneticwaves of specific wavelengths.

According to the tenth embodiment of the invention, there is provided ascreen comprising:

first particles regularly aligned to reflect an electromagnetic wave ofa first wavelength; and

second particles regularly aligned to reflect an electromagnetic wave ofa second wavelength different from said first wavelength,

wherein said first particles and said second particles are different indiameter.

In the ninth and tenth aspects of the invention, the electromagneticwave is typically visible light. In this case, within the extentconsistent to its nature, the foregoing matters already referred to inconjunction with the first to eighth aspects of the invention are hereagain applicable.

According to the invention having the above-summarized configurations,it is possible to selectively reflect only light of specific wavelengthsby means of the photonic crystals, particles or dielectric multi-layeredfilms and to absorb the other part of light of other wavelengths byusing an absorption layer, for example.

Moreover, by regularly aligning particles in a self-systemized manner,desired particle layers can be formed easily.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A through 2B are schematic diagrams for explaining the principleof screens according to the invention;

FIG. 3 and FIG. 4 are schematic diagrams that show reflection spectrumsof multi-layered films;

FIG. 5 is a schematic diagram that shows a reflection spectrum ofregularly aligned particles;

FIGS. 6A through 6C are schematic diagrams for explaining a close-packedstructure;

FIG. 7 is a schematic diagram that shows spectrums of scattered light ofregularly aligned particles;

FIG. 8 is a schematic diagram for explaining a reason why light of aspecific wavelength is reflected;

FIGS. 9A through 10 is a schematic diagram that shows a model used forcalculation of a light field of particles;

FIGS. 11A through 31B are schematic diagrams that show results ofcalculation of light fields of particles;

FIG. 32 is a schematic diagram that shows a model used for calculationof a light field of particles for green reflection;

FIGS. 33A through 39B are schematic diagrams that show results ofcalculation of light fields of particles for green reflection;

FIG. 40 is a schematic diagram that shows a model used for calculationof a light field of particles for blue reflection;

FIGS. 41A through 47B are schematic diagrams that show results ofcalculation of light fields of particles for blue reflection;

FIG. 48 is a schematic diagram that shows a relation between thediameter of a silica particle and the wavelength inviting Braggreflection;

FIG. 49 is a schematic diagram that shows a model used for calculationof a light field of particles for reflection of three primary colors;

FIGS. 50A through 54B are schematic diagrams that show results ofcalculation of light fields of particles lateral reflection of threeprimary colors;

FIG. 55 is a cross-sectional view of a screen according to the firstembodiment of the invention;

FIG. 56 is a schematic diagram for explaining a manufacturing method ofthe screen according to the first embodiment of the invention;

FIGS. 57A through 60D are schematic diagrams for explaining a moreconcrete manufacturing method of the screen according to the firstembodiment of the invention;

FIG. 61 is a schematic diagram that shows how light spreads bydeflection;

FIGS. 62A through 65C are schematic diagrams that show results ofcalculation of light fields of particles;

FIGS. 66 and 67 are schematic diagrams that show how light spreads bydeflection;

FIG. 68 is a schematic diagram that shows a model used for calculationof a light field of particles;

FIGS. 69A through 75B are schematic diagrams that show results ofcalculation of light fields of particles;

FIG. 76 is a schematic diagram that shows extension of reflected lightas extension of a far-field pattern;

FIGS. 77A and 77B are schematic diagrams for explaining results ofinclination of the crystal axis;

FIG. 78 is a schematic diagram that shows a reciprocal lattice space;

FIG. 79 is a schematic diagram that shows relations between inclinationof the crystal axis and wavelengths satisfying the Bragg's condition;

FIG. 80 is a schematic diagram that shows an example of a structurerelaxing the directivity;

FIG. 81 is a schematic diagram that shows a reciprocal lattice space;

FIGS. 82 and 83 are schematic diagrams that show reflection spectrums ofdielectric multi-layered films;

FIGS. 84 through 87 are schematic diagrams that show measured spectrumsof light emitted from an LCD projector;

FIGS. 88 through 91 are schematic diagrams that show measured spectrumsof light emitted from a DLP projector;

FIG. 92 is a schematic diagram that shows a chromaticity diagram;

FIG. 93 is a cross-sectional view that shows a screen according to thesecond embodiment of the invention;

FIG. 94 is a cross-sectional view that shows a screen according to thethird embodiment of the invention;

FIG. 95 is a cross-sectional view that shows a screen according to thefourth embodiment of the invention;

FIG. 96 is a cross-sectional view that shows a screen according to thefifth embodiment of the invention;

FIGS. 97A through 97C are plan views that show parallel-to-substratepatterns of alignment of particle layers for reflection of three primarycolors on the screen according to the fifth embodiment of the invention;

FIG. 98 is a cross-sectional view that shows a screen according to thesixth embodiment of the invention;

FIG. 99 is a cross-sectional view that shows a screen according to theseventh embodiment of the invention;

FIG. 100 is a cross-sectional view that shows a screen according to theeighth embodiment of the invention;

FIG. 101 is a cross-sectional view that shows a screen according to theninth embodiment of the invention;

FIG. 102 is a cross-sectional view that shows a screen according to thetenth embodiment of the invention;

FIG. 103 is a cross-sectional view that shows a screen according to theeleventh embodiment of the invention; and

FIGS. 104 and 105 are schematic diagrams that show an image displaysystem according to the twelfth embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be explained below. In all figuresshowing the embodiments, common or equivalent components are labeledwith common reference numerals.

As shown in FIGS. 1A and 1B, a screen capable of lowering the luminancelevel of black-displaying portions can be realized by combining areflector and a light absorption layer so as to reflect light of only aspecific wavelength and absorb light of the other wavelengths. Thescreen shown in FIG. 1A is characterized in a high wavelengthselectivity whereas the screen shown in FIG. 1B is characterized in asimple structure.

FIGS. 2A and 2B show specific examples of structure for reflecting lightof a specific wavelength exclusively. The structure shown in FIG. 2A ismade of a regular alignment of particles optimized in size beforehand ona substrate to selectively reflect light of a wavelength satisfying theBragg's condition (λ=2nΛ/m, λ: wavelength of the incident light; n: moderefractive index; Λ: structural cycle; m: order). The structure shown inFIG. 2B is made by forming a multi-layered film on a substrate byalternately stacking films having the reflective indices n₁ and n₂ (≠n₁)to selectively reflect light of specific wavelengths by using theinterference effect.

First explained is a result of estimation of the reflection spectrums ofthe multi-layered film by the effective Fresnel's coefficient method.This multi-layered film is an alternative lamination of two kinds ofdielectric films different in refractive index, each stacked to thethickness of mλ₀/4n relative to the refractive index of each. Ingeneral, m is an integer not smaller than 1, but here, it is 1. λ₀ isthe specific wavelength of light. The result is shown in FIG. 3. Thecalculation is conducted here by setting the refractive index of onedielectric film as n=1.2, the refractive index of the other kind ofdielectric film as n=1.8 and λ₀=520 nm. This result shows that thereflectance increases as the cycle of the multi-layered film increasesfrom 1 to 5 and that a reflectance not less than 90% is obtained whenthe film is stacked to five cycles. It is also understood that the fullwidth at half maximum is as wide as ˜200 nm.

FIG. 4 shows a result of calculation about wavelengths λ₀=490 nm (blue),λ₀=520 nm (green) and λ₀=650 nm (red) of three primary colors under thecondition of five cycles. It is understood from the result that, for anyof the wavelengths of three primary colors, because of a wide full widthat half maximum of the peak, peaks overlap, but light of a specificwavelength can be reflected to a certain extent.

Although the manufacturing method will be explained later in greaterdetail, FIG. 5 shows measured reflection spectrums of silica particles(diameter D=280 nm) regularly aligned into a close-packed structure byself-organization. In this measurement, however, white light isintroduced normally to the particle layer, and normally reflected lightis calculated. Through observation by scanning electron microscopy(SEM), it is assumed that particles form a close-packed structure offace-centered cubic lattice or a close-packed hexagonal lattice as shownin FIGS. 6A, 6B and 6C by self-organization. In FIG. 5, a peak isobserved near the wavelength 625 nm. It is also observed that themaximum reflectance is relatively as low as ˜54% and the full width athalf maximum is as narrow as ˜30 nm. This reflection is Bragg'sreflection by regularly aligned particles. In this manner, the Bragg'sreflection occurs due to the periodical structure of the same unitperiod (<1 μm) as the wavelength order of visible light.

The Bragg's reflection will be explained below in greater detail.

In a close-packed structure, there are three patterns of alignment A, Band C as shown in FIG. 6A. In case of a face-centered cubic lattice, thepatterns are stacked in the order of A, B, C, A, B, C, . . . as shown inFIG. 6B. If the particle diameter is D=280 nm, the period is Λ=727.5 nm.In case of a hexagonal close-packed lattice however, since the patternsare stacked in the order of A, B, A, B, . . . as shown in FIG. 6C, theperiod is Λ=485.0 nm. Taking them into account, wavelengths satisfyingthe Bragg's condition (λ=2nΛ/m) can be estimated by calculation as shownin Table 1. The mode refractive index n employed here was ˜1.3.

TABLE 1 Face-centered Hexagonal Close- Cubic Lattice packed Lattice m λ(nm) λ (nm) 1 1891 1261 2 946 630 3 630 420 4 473 315

The calculation gives two candidates as values nearest to 625 nm. Thatis, it is shown that intensive peaks observed in the reflectionspectrums are the tertiary Bragg's reflection of the face-centered cubitlattice or the secondary Bragg's reflection of the hexagonalclose-packed lattice. This means that Bragg's reflection has beenconfirmed by regular alignment of particles stacked byself-organization.

FIG. 7 shows measured spectrums of light scattered by the particlelayers, which were obtained by inclining the sample surface by 20°. Inthis case, a reciprocal pattern (dip structure) is confirmed, in whichalmost no light of wavelengths near 625 nm is reflected. This indicatesthat scattered light is suppressed by strong Bragg's reflection. Thisphenomenon can be explained as follows. As shown in FIG. 8, light ofwavelengths near 625 nm suffers strong Bragg's reflection near thesurface of the particle layer, and cannot travel deeper. Therefore, itsscattering is weak, and only the Bragg's reflection is stronglyreceived. On the other hand, light of wavelengths other than 625 nm,which is free from Bragg's reflection and can travel deeper, results inbeing scattered.

Additionally, using the particles aligned into the close-packedstructure referred to above, wavelength bands exhibiting strongreflection of light were estimated by light field calculation using aMaskwell equation. Note here that, while each actual particle had around shape as shown in FIG. 9A, calculation was carried out byapproximating each particle into an approximately square shape as shownin FIG. 9B. In the calculation, lateral (x) and vertical (y) intervalsof square-shaped particles were assumed to be equal to those ofround-shaped particles (x=242 nm, y=280 nm). Both were equalized also infilling coefficient. Refractive index of the particles was assumed to ben=1.36, and taking account of the thickness of the samples, theirstacking cycles were assumed to be 30 cycles (FIG. 10). Results of thecalculation are shown in FIGS. 11 through 19. Density distribution oflight was calculated here by dividing the light entering into theparticle layer from the left in the figure into the part traveling inthe forward direction (from left to right in the figure as beingindicated as “FORWARD” as well) and the other part traveling in theopposite direction (from right to left in the figure as indicated as“BACKWARD” as well). Note, however, that these density distributiondiagrams were prepared by first printing color images with a colorprinter and then copying them with a black-and-white copying machine andthat densities do not always correspond to densities of light (also inthe description made below). Furthermore, because of the constraint ofthe sheet size, the images are downsized in the lateral direction. Theresults shown in FIGS. 11 through 19 demonstrate that in the light ofthe wavelengths 470 nm, 500 nm, 525 nm, 540 nm, 580 nm, 600 nm, 645 nmand 675 nm, only the part traveling in the forward direction stronglyexists, and the light reaches the right end of the particle layer andthen exits rightward from its surface. In contrast, the part of thelight traveling in the opposite direction exists merely inside the bulk,and even though it reaches the left end of the particle layer, almost nolight exits leftward from its surface. However, as shown in FIG. 17, inthe light of the wavelength 625 nm, the part traveling in the oppositedirection is strongly generated near the surface, and light intensivelyexits leftward from is surface. It is also appreciated that, because ofintensive light traveling in the opposite direction, the light in theforward direction does not enter ahead from the surface to a depthbeyond 8 through 15 cycles. Especially, 11 cycles or so are consideredto be the boundary. These results coincide with the result of anexperiment, which demonstrates that intensive reflection occurs withlight of wavelengths near 625 nm. FIGS. 20 through 31 show results ofdetailed analysis using wavelengths near 625 nm. These resultsdemonstrate that reflection occurs with wavelengths in the range from605 nm to 632 nm, and they coincide well with the result of theexperiment that the full width at half maximum of the reflectance peakwas as narrow as ˜30 nm (FIG. 5). A possible reason of the narrow fullwidth at half maximum of the reflectance peak by particles as comparedwith the multi-layered film may be that, in case of particles, Bragg'sreflection occurs also in the lateral direction and leads to a strongeffect of confinement. Additionally, in case of the light having, thewavelength of 625 nm causing Bragg's reflection, the light cannot enterbeyond 8 through 15 cycles from the surface, and this corresponds to thefact that scattered light is suppressed.

Next explanation is directed to how green and blue light is reflected.Since the diameter D of each particle and the wavelength λ of lightreflected by the particles are approximately proportional, if thewavelength of light to be reflected is λ₀, then the diameters D=235 nmand D=212 nm are obtained respectively for green (λ₀=525 nm) and blue(λ₀=475 nm) from the relation λ₀=625 nm relative to D=280 nm. Forrespective cases, calculation of light fields was conducted. A model ofgreen reflection is shown in FIG. 32, and results of the calculation areshown in FIGS. 33 through 39. A model of blue reflection is shown inFIG. 40, and results of the calculation are shown in FIGS. 41 through47. These results demonstrate that strong reflection occurs only withthe wavelength of 525 nm for green reflection and 475 nm for bluereflection, respectively, and that the light travels deep approximatelyto 8 through 15 cycles similarly to red reflection.

Since the diameter D of the particle and the wavelength λ aresubstantially proportional, they exhibit the relation shown in FIG. 48.Here is shown that the diameter is D=202˜224 nm for blue reflection,D=224˜251 nm for green reflection and D=269˜314 nm for red reflection.Especially for pure three primary colors on a chromaticity diagram,D=208˜217 nm with λ₀=475±10 nm for blue reflection, D=224˜237 nm withλ₀=515±15 nm for green reflection, and D=278˜305 with λ₀=650±30 nm.

It is appreciated from those results that it is possible to reflectlight only of the three primary colors and transmit light of the otherwavelengths by stacking 11 cycles of particles layers for red reflectionon a substrate, 11 cycles of particles layers for green reflectionthereon, and 11 cycles of particle layers for blue reflection thereon.They were similarly estimated by calculation of light fields. The modelused therefor is shown in FIG. 49, and results of calculation are shownin FIGS. 50 through 54. These results demonstrate that strong reflectionoccurs in the portions of particle layers for blue reflection, greenreflection and red reflection, respectively, when the wavelength is 475nm, 525 nm and 623 nm and that the light does not travel deeper. Incontrast, with wavelengths such as 590 nm and 555 nm other than those ofthree primary colors, almost no reflection occurs, and the lighttherefore reaches to the right end of the particle layer for redreflection and exits therefrom rightward. Therefore, light of colorsother than three primary colors can be cut efficiently by locating alight-absorbing material at a deep portion, e.g. by locating alight-absorbing material as the substrate.

Taking it into consideration, the first embodiment of the inventionconfigures the screen to have a cross-sectional structure shown in FIG.55. That is, the screen is made by stacking 11 cycles of particleslayers 2 of D=280 nm for red reflection on the substrate 1, 11 cycles ofparticles layers 3 of D=234.5 nm for green reflection thereon, and 11cycles of particle layers 4 of D=212 nm for blue reflection thereon. Inany of the particle layer 2 through 4, particles 5 are aligned into aclose-packed structure. Particles of the particle layers 2 to 4 may besilica particles. Used as the substrate 1 is any material that canabsorb light of wavelengths other than three primary colors. Forexample, a black substrate of carbon may be used. Thickness of thesubstrate 1 is in the range from 20 μm to 500 μm, and it may be around50 μm. If the thickness of the substrate 1 is around 50 μm, then thescreen is unlikely to break, and at the same time, because of a highflexibility, it can be readily rolled. The area of the screen may bedetermined appropriately in accordance with its use.

The screen shown in FIG. 55 can be easily manufactured by using aself-organized technique, for example. That is, as shown in FIG. 56, forexample, if a water solution 6 containing dispersed particles 5 is usedto let the particles 5 slowly accumulate in the water solution 6, theparticles 5 regularly align by its self-organization. Thus, by usingthis self-organized technique, particle layers 2 through 4 can besequentially stacked in a regular alignment on the substrate 1, therebyto manufacture the screen.

The manufacturing method of the screen will be explained below ingreater detail. Generally used methods for manufacturing a screen ofthis kind include natural precipitation (for example, Masuda, et al.(2001) Material Integration 14, 37–44) and immerse-pull-upsingle-layered particle film manufacturing method (single-layeredparticle film-pull-up) (for example, Nagayama (1995) Powder Technology32, 476–485). In a natural precipitation process, a low-concentratedparticle solution is poured onto a substrate, or a substrate isvertically dipped in a low-concentrated particle solution. Then,particles precipitating onto the substrate are crystallized on thesubstrate in a self-organized manner by vaporization of the solvent.Natural precipitation is a method of obtaining a thin film ofthree-dimensional crystals of particles on a substrate through thisprocess. A problem with this method is the need of at least severalhours for vaporization of the solvent. It therefore takes a long time todry the substrate, and since the solvent vaporizes from the substrateunevenly along the surface, the thin film becomes uneven in thicknessalong the surface especially upon making a large-area thin film ofcrystals as large as several cm². On the other hand, single-layeredparticle film pull-up technique is a method using a process of forming athin film of two-dimensional crystals in a single layer of particles byimmersing a substrate into a low-concentrated particle solution andpulling it up into the air. In this method, a thin film ofthree-dimensional crystals having any desired thickness is obtained byrepeating that process to stack thin films each of a single layer ofparticles. Problems with this method are that the process of stackingfilms each of a single layer of particles is complicated and needs along time for fabrication and that the pull-up speed must be kept lowfor ensuring two-dimensional uniform crystallization along the surface.In case of a large-area thin-film as large as several cm², long-timecontrol is required to keep the air-liquid interface meniscus in order,and this is not easy.

Taking them into consideration, here is used a pull-up-and-rotateprocess as the method of significantly reducing the fabrication time bymaking use of both the fabrication of three-dimensional crystals bynatural precipitation and alleviation of uneven thickness along thesurface by single-layered particle film pull-up technique. Although thesingle-layered particle film pull-up technique can make a thin film ofonly a single-layered two-dimensional alignment of particles in onecycle of immersion and pull-up, the pull-up-and-rotate method can make athin film of three-dimensional alignment of crystals in one cycle ofimmersion and pull-up similar to the single-layered particle pull-uptechnique by using a high-concentrated particle solution. As a result,this method can make three-dimensional crystals like that by naturalprecipitation. Then, by rotating the substrate, unevenness of thethickness along the surface can be reduced like the single-layeredparticle pull-up method. Additionally, this method can remarkably reducethe time required for the fabrication process.

In this pull-up-and-rotate method, when a substrate is immersed into ahigh-concentrated particle solution and pulled up therefrom into theair, the thickness becomes uneven because it takes a long time for thesubstrate to dry and particles concentrate to wetter portions.Unevenness of the thickness occurs from a lower portion in theperpendicular direction and right and left ends in the horizontaldirection of the substrate. Thus the substrate was rotated in parallelto its surface before immersion, during immersion or immediately afterpulling it up to control the wetness. As a result, the unevenness of thethickness was alleviated, and a thin film having a uniform thicknessthroughout the entire extension thereof was obtained.

Referring to FIGS. 57 through 60, the pull-up-and-rotate method isexplained below in a more practical manner.

As shown in FIG. 57A, first prepared is a solution vessel 7 containing aparticle solution 8 of a high concentration (for example, from 2 weight% to 50 weight %). After that, as shown in FIG. 57B, the substrate 1 islowered from above the solution vessel 7 into the particle solution 8.Then, after the substrate 1 is pulled up as shown in FIG. 57C at a highspeed (for example, in the range from 30 μm/s to 3 m/s), it is naturallydried in the air as shown in FIG. 57D.

In these steps, the particle solution 8 once adhering on the substrate 1moves down due to the gravity as it dries. Therefore, distribution ofparticles is localized to a lower portion of the substrate 1, and afterthe substrate 1 dries, the thin film obtained results in producing anuneven distribution having a thick lower portion and a thin upperportion in the perpendicular direction within the extension thereof. Inthe perpendicular direction, the unevenness in thickness in theextension of the thin film can be prevented by carrying out thefollowing steps.

As shown in FIG. 58A, the substrate 1 after being dried in the stepshown in FIG. 57D is rotated upside down by 180° in parallel to itsplane. After that, as shown in FIG. 58B, the substrate 1 is lowered fromabove the solution vessel 7 into the particle solution 8. After that, inthe same manner as already explained, the step of high-speed pull-up ofthe substrate 1 (FIG. 58C) and natural drying in the air (FIG. 58D) arecarried out. As a result, although the thickness of the particle layerhas the distribution including a locally thick portion on a lower partof the substrate 1 and a locally thin portion on an upper part of thesubstrate 1, since it is opposite from the thickness distribution of theparticle layer stacked earlier, the thickness distribution of the entiresubstrate 1 is uniformed in the perpendicular direction. Also when theupside-down rotation of the substrate 1 is carried out during immersionor immediately after pull-up instead of rotation before immersion, thesame effect is obtained.

In order to uniform the thickness distribution on the entire substrate 1also in the horizontal direction, steps similar to those of FIGS. 57 and58 are carried out.

That is, as shown in FIG. 59A, the substrate 1 after being dried in thestep shown in FIG. 58D is rotated by 90° clockwise in parallel to itsplane. After that, in the same manner as already explained, immersion ofthe substrate 1 into the particle solution 8 (FIG. 59B), quick pull-upof-the substrate 1 (FIG. 59C) and natural drying in the air (FIG. 59D)are carried out.

After that, as shown in FIG. 60A, the substrate 1 after being dried inthe step shown in FIG. 59D is rotated upside down by 180°. Subsequently,in the same manner as already explained, immersion of the substrate 1into the particle solution 8 (FIG. 60B), quick pull-up of the substrate1 (FIG. 60C) and natural drying in the air (FIG. 60D) are carried out.

By the method explained above, a wide-area particle thin filmcrystallized with no visible unevenness throughout the extension thereofcan be obtained in a short time.

It could be possible to employ an alternative way of reducing thethickness unevenness by laying the substrate 1 horizontally, therebyuniforming the liquid-holding ability of the substrate 1 throughout itsentire surface, and thereafter drying it. However, as far as theInventor actually tried it, the liquid-holding ability could not be keptuniform, and unevenness of thickness was produced within the planethereof.

Here is explained a result of comparison in unevenness of thicknessbetween a particle thin film prepared by natural precipitation and aparticle thin film prepared by the pull-up-and-rotate method.

In this comparison, silica particles having the diameter of 280 nm(Product KE-P30 by Nippon Catalyst) were used as the particles, purewater as the solvent, and commercially available plasma-washed aluminumfoil (in form of a rectangle sized 26 mm each shorter side and 76 mmeach longer side) as the substrate.

In preparation of a sample by natural precipitation, the quantity of 20μl of water solution containing 20 weight % of silica was poured andspread on one surface of a substrate. The substrate was maintainedhorizontal and dried for three days within a sample case of a resin.

In preparation of a sample by the pull-up-and rotate method, a substratewas immersed in a water solution containing 20 weight % of silica, withthe longer sides of the substrate being oriented perpendicularly, thenpulled up vertically at the speed of 10 m/s keeping that posture, anddried. After it was dried, the substrate was rotated upside down, andsimilarly immersed, pulled up and dried. Subsequently, the substrate wasrotated by 90° in parallel to its surface, then immersed with itsshorter sides being oriented perpendicularly, pulled up vertically atthe speed of 10 mm/s while keeping the posture, and dried. After it wasdried, the substrate is rotated upside down, and similarly immersed,pulled up and dried. In this manner, four cycles of immersion andpull-up steps were repeated.

As a result of visual comparison of both samples, unevenness inthickness was smaller in the sample by the pull-up-and-rotate methodthan in the sample by natural precipitation. In addition, both exhibitedBragg's reflection, and silica particles were confirmed to formthree-dimensional crystals.

Thickness of each prepared thin film were measured at five points alonga line connecting its opposite shorter sides (center point, and pointsdistant by 10 mm and 20 mm from the center point). Thickness wasmeasured by optical measurement as the perpendicular distance betweenthe surface of the aluminum foil as the substrate and the surface ofeach prepared thin film. Its result is was as follows.

Natural Precipitation:

-   -   Average value: 14.8 μm    -   Standard difference: 3.1 μm

Pull-up-and-Rotate Method:

-   -   Average Value: 9.9 μm    -   Standard Difference: 0.6 μm

It has been confirmed from the difference between those standarddifferences that the variance of the film thickness by thepull-up-and-rotate method is much smaller than that by naturalprecipitation. Through four cycles of the pull-up-and-rotate process, asilica thin film of three-dimensional crystals made up of about 35layers with small unevenness in thickness could be obtained.

As explained above, according to the first embodiment, since it ispossible to reflect the light of three primary colors exclusively whileabsorbing the light of other wavelengths on the part of the substrate 1,a screen lowering the luminance level of black-displaying portions canbe obtained. In this case, even when external light irrelevant to imagesenters into the screen, it is cut because of the difference inwavelength, and deterioration of the contrast is prevented. Especiallywhen images are formed with light having a narrow full width at halfmaximum of the emission peak and excellent in color purity, such aslight from a semiconductor laser or LED, it is possible to maintain ahigh contrast and lower the luminance level of black-displaying portionsby efficiently selectively reflecting only the light for images andcutting light of other wavelengths. Therefore, deterioration of imagesdoes not occur even in circumstances other than dark rooms. Furthermore,even when light with a wide spectral full width at half maximum isprojected from, for example, a liquid crystal projector, since the lightis selectively narrowed in wavelength, the chromatically reproduciblerange on the chromaticity diagram is enlarged, and the color purity isalso improved.

Next explained is a method of expanding the far field pattern (FFP) ofreflected light by making use of the diffraction effect.

As shown in FIG. 61, in general, if the size of an object issufficiently small in the direction normal to the direction of incidenceof light, then the light is diffracted and spread by the object. Takingit into consideration, by making up the screen of an aggregate ofparticles, it will be possible to make an extension of FFP of reflectedlight with the diffraction effect by the particles. This corresponds tolaterally extending the lattice point on the space of the reciprocallattice. Taking it into account, reflected waves in case of the lateralsize being 22 cycles, 16 cycles and 11 cycles were calculated in a widearea (100 μm×30 μm). Its results are shown in FIGS. 62 through 65. It isappreciated from these results that FFP becomes wider as the lateralcycles decreases. More specifically, although FFP is as narrow as ˜8°when the lateral size is 22 cycles, FFP is enlarged as the cyclesdecreases, to ˜11° with 16 cycles and ˜17° with 11 cycles.

In case images are displayed on a screen in a big site like a theatre,the angle of the field of view may be relatively narrow, and brightnessis rather requested. In this case, it is possible to relatively narrowFFP as narrow as 10˜17° to give a certain directivity and to therebyincrease the density of light, that is, to make the screen brighter.

Next explained is a method of expanding FFP of reflected light by makinguse of refraction.

In order to provide FFP of reflected light with an extension byrefraction, it will be possible to shape the aggregate of particles intoa structure having a horizontal surface and sloping surfaces as shown inFIG. 66 or to shape the surface of the aggregate of particles into asurface-curved structure as shown in FIG. 67. In the example shown inFIG. 66, reflected light is emitted obliquely only in a specificsurface, but in the example shown in FIG. 67, reflected light is emittedin any directions in accordance with the curved surface.

Calculation was carried out by changing the angle θ of the slopingsurface of the aggregate of particles relative to the normal direction(direction of the crystal axis) as shown in FIG. 68. In thiscalculation, wavelength of the incident light was 625 nm, and diameterof each particle was 280 nm (these are conditions causing Bragg'sreflection when light enters normally to the horizontal surface (leftend surface in FIG. 68). Its results are show in FIGS. 69 through 73. Itis appreciated from these results that almost no Bragg's reflectionoccurred and light could pass through even when light impinges surfacesof θ=14.4° and θ=58.2°. In contrast, light was reflected on slopingsurfaces of θ=70.2°, θ=75.7° and θ=78.9°.

Furthermore, results of BACKWARD on the wider reflection side are shownin FIGS. 74 and 75. It is appreciated from these results that obliquereflection did not occur when θ=14.4° and θ=58.2° but occurred whenθ=70.2°, θ=75.7° and θ=78.9°. FIG. 76 is a graph that shows a resultthereof as FFP. This result demonstrates that a peak appeared near 35°.It is appreciated from these results that the method of refraction canprovide FFP with an extension up to 70° if a sloping surface is formedin the range of θ=90˜70°.

Next explained is the case where the crystal axis inclines from thedirection of incidence of light as shown in FIG. 77. In this case, thewavelength satisfying the Bragg's condition shifts. If the wavelengthupon vertical incidence (the direction of incidence is parallel to thecrystal axis) is λ₀, the wavelength satisfying the Bragg's conditionbecomes λ(θ)=λ₀ sin θ. This means that a lattice point rotates about theorigin in a space of reciprocal lattice due to deviation of thedirection of incidence of light and results in failing to ride on thesurface of the same Ewald's sphere (sphere having the radius 1/λ). Aresult of calculation taking this effect into consideration is shown inFIG. 79. This result demonstrates that Bragg's reflection occurs in therange of θ=77.4˜90° when the full width at half maximum of the spectrumis 30 nm. In case of θ=77.4°, light will be reflected in the directioninclined by 2θ=25.6° from the normal direction. However, if the axis isinclined in the opposite direction, namely, by θ=−77.4°, then FFP isenlarged to FFP=51.6° in total.

The method of using refraction and the method of inclining the crystalaxis are suitable for use in projection onto a screen in a narrow spacein a private house, for example, because high directivity will makeimages invisible from offset positions.

In order to moderate the directivity, the aggregate 9 of particles mayhave undulation as shown in FIG. 80.

Next explained is the wavelength selectivity of the screen.

The wavelength selectivity can be also explained by using the space of areciprocal lattice. That is, as shown in FIG. 81, in case the size inthe incident direction of light is small, the lattice point in thereciprocal lattice space extends in that direction. It results inexistence of a number of Ewald's spheres that intersect the latticepoint, and results in expanding the range of wavelengths λ thatsatisfying the Bragg's condition. Taking dielectric multi-layered filmsstacking five layers and ten layers, respectively, their reflectionspectrums were calculated by the effective Fresnel's coefficient method.Its results are shown in FIGS. 82 and 83. It is appreciated from theseresults that the full width at half maximum is about 200 nm in thefive-layered film, but it is as narrow as 50 nm in the ten-layered film.However, it is insufficient to simply increase the layers for thepurpose of improving the wavelength selectivity, but it is necessary toincrease the effective size for light. Even if stacking layers, whichreflect by 100% in combination of several layers, up to 100 layers orso, the effective size is only those several layers, the wavelengthselectivity remains bad. Therefore, it is desirable to minimize thereflection efficiency of individual diffraction gratings by particles tomake up a structure in which diffraction takes place over many layers.

As explained above, the use of the screen according to the firstembodiment narrows the full width at half maximum of the spectrum ofeach of three primary colors. It is explained below that it improves thecolor purity and expands the reproducible range on the chromaticitydiagram.

FIGS. 84 through 87 and FIGS. 88 through 91 show measured spectrums oflight emitted from an LCD (Liquid Crystal Display) projector and a DLP(Digital Light Processing) projector, respectively. FIGS. 84 and 88 showspectrums measured when displaying white, FIGS. 85 and 89 show spectrumswhen displaying blue, FIGS. 86 and 90 show spectrums measured whendisplaying green, and FIGS. 87 and 91 show spectrums measured whendisplaying red. Since a color filter is used to select wavelengths, boththe LCD projector and the DLP projector emit light in which the fullwidth at half maximum of the spectrum of each primary color is as wideas 60˜100 nm. If here is used an ordinary screen, no change occurs inthe full width at half maximum even when light is reflected, and thefull width at half maximum of the spectrum determines the colorreproducibility. In contrast, in case the screen according to the firstembodiment is used, even when the spectrum of each primary color in thelight emitted from the projector has a wide full width at half maximum,the predetermined wavelength is selected when the light is reflected bythe screen, and the full width at half maximum is narrowed to 30 nm. Atthat time, the reproducible range of colors on the chromaticity diagramis enlarged, and the color reproducibility is improved. FIG. 92 shows iton the chromaticity diagram. Although the color reproducible range isnarrow with DLP and LCD, the use of the screen according to the firstembodiment enlarges the range and simultaneously improves the colorreproducibility.

Next explained is a screen according to the second embodiment of theinvention. FIG. 93 shows this screen.

As shown in FIG. 93, in the screen according to the second embodiment, adiffusion film 10 is located on the top surface of the particle layer 4.The diffusion film 10 is used to diffuse light and to protect the screensurface. That is, by diffusing the light reflected by the screen withthe diffusion film 10, it is possible to moderate the directivity and touniform the luminosity throughout the screen. In other words, so-calledhot spots can be eliminated. The diffusion film 10 can also preventexfoliation of particles due mechanical damage.

A material transparent in the region of visible light and diffusinglight is preferably used as the diffusion film 10. To diffuse light, adistribution of different refractive indices may be made in parallel tothe film surface, or undulations may be made on the film surface.Specific materials of the diffusion film 10 include light-diffusingpolyethylene films (inherently having a distribution of refractiveindices in parallel with the film surface when manufactured),polycarbonate films, polyethylene terephthalate films and polyvinylchloride films whose surfaces are treated to form undulations. Thicknessof the diffusion film 10 is normally not larger than 5 mm, andpreferably not larger than 1 mm.

To add the diffusion film 10, after the particle layers 2 through 4 arestacked on the substrate 1, the diffusion film 10 may be spread andbonded on the surface of the particle layer 4 under application of atensile force. Alternatively, the diffusion layer 10 may be bonded withan adhesive previously coated on the back surface of the diffusion film10. Furthermore, for the purpose of improving its optical property, thesurface of the diffusion film 10 may be processed by ¼ wavelengthcoating for preventing reflection. In this case, it is important thatthe coating material has a lower refractive index than that of the filmmaterial. More specifically, a SiO₂ glass film is coated to a thicknessof ˜100 nm by mechanical coating or vapor deposition.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, and their detailed explanation isomitted.

Next explained is a screen according to the third embodiment of theinvention. FIG. 94 shows this screen.

As shown in FIG. 94, in the screen according to the third embodiment, amicro lens film 11 having formed a two-dimensional array of micro lensesis located on the top surface of the particle layer 4. The micro lensesof the micro lens film 11 may be any of convex lenses, concave lenses ortheir mixture. By diffusing light reflected back from the screen withthe micro lens film 11, it is possible to moderate the directivity andto uniform the luminosity throughout the screen and thereby eliminatethe hot spots. The micro lens film 11 can also prevent exfoliation ofparticles due mechanical damage.

Any material transparent in the region of visible light is basicallyusable as the micro lens film 11. For example, polycarbonate,polyethylene terephthalate and polyvinyl chloride are acceptable. It issufficient for the micro lenses of the micro lens film 11 to be equal orsmaller than the pixel size, and lenses sized about 0.1 mm in diameter,for example, may be closely packed along the plane. Furthermore, for thepurpose of improving its property, the surface may be processed by ¼wavelength coating for preventing reflection. In this case, it isimportant that the coating material has a lower refractive index thanthat of the lenses of the micro lens film 11. More specifically, a SiO₂glass film may be coated to a thickness of ˜100 nm by mechanical coatingor vapor deposition.

The same method as that used in the second embodiment is used here againto add the micro lens film 11.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

Next explained is a screen according to the fourth embodiment of theinvention. FIG. 95 shows this screen.

As shown in FIG. 95, in the screen according to the fourth embodiment, amicro prism film 12 having formed a two-dimensional array of microprisms is located on the top surface of the particle layer 4. Bydiffusing light reflected back from the screen with the micro prism film12, it is possible to moderate the directivity and to uniform theluminosity throughout the screen and thereby eliminate the hot spots.The micro prism film 12 can also prevent exfoliation of particles duemechanical damage.

Any material transparent in the region of visible light is basicallyusable as the micro prism film 12. For example, polycarbonate,polyethylene terephthalate and polyvinyl chloride are acceptable. It issufficient for the micro lenses of the micro prism film 12 to be equalor smaller than the pixel size, and prisms sized about 0.1 mm indiameter, for example, may be closely packed along the plane.Furthermore, for the purpose of improving its property, the surface maybe processed by ¼ wavelength coating for preventing reflection. In thiscase, it is important that the coating material has a lower refractiveindex than that of the prisms of the micro prism film 12. Morespecifically, a SiO₂ glass film may be coated to a thickness of ˜100 nmby mechanical coating or vapor deposition.

The same method as that used in the second embodiment is used here againto add the micro prism film 12.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

Next explained is a screen according to the fifth embodiment of theinvention. FIG. 96 shows this screen.

In the first to fourth embodiments already explained, the red-reflectingparticle layer 2, green-reflecting particle layer 3 and blue-reflectingparticle layer 4 are stacked in the vertical direction (direction normalto the substrate) on the substrate 1. In the fifth embodiment, however,the particle layers 2 through 4 are formed to align in the lateraldirection (the direction in parallel to the substrate).

That is, as shown in FIG. 96, in the screen according to the fifthembodiment, the red-reflecting particle layer 2, green-reflectingparticle layer 3 and blue-reflecting particle layer 4 are arrayed sideby side on the substrate 1.

An example of parallel-to-substrate configurations of the particlelayers 2 through 4 and their parallel-to-substrate arrangement patternare shown in FIG. 97. In FIG. 97A, the particle layers 2 through 4 eachhaving a stripe-shaped parallel-to-substrates configuration are arrayedalternately. Width of each particle layer 2, 3, 4 may be equal to orsmaller than ⅓ of the pixel size. In the example shown in FIG. 97B,particle layers 2, 3, 4 each having a rectangular parallel-to-substrateshape are arranged in a checkered pattern. Size of each rectangularparticle layer 2, 3, 4 may be equal to or smaller than ⅓ of the pixelsize. In the example shown in FIG. 97C, particle layers 2, 3, 4 eachhaving a square parallel-to-substrate shape are arranged in a checkeredpattern. Size of each square particle layer 2, 3, 4 may be equal to orsmaller than ⅓ of the pixel size.

For forming the particle layers 2 through 4 on the substrate 1,particles for respective colors may be selectively, locally coated onthe substrate 1 in an ink-jet manner, or may be selectively, locallycoated by screen printing or gravure printing. It is also possible touse masks having openings corresponding to patterns of respectiveparticle layers 2 through 4 to coat particles for respective colors inthree steps of coating using the masks, respectively.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the fifth embodiment, since the particle layers for threeprimary colors are arranged in the lateral direction on the substrate 1,the entire particle layers 2 through 4 make a thickness smaller in thevertical direction than that of the particle layers vertically stackedon the substrate 1, and thereby decrease the loss by scattering oflight, for example, to ensure efficient absorption of light.

Next explained is a screen according to the sixth embodiment of theinvention. FIG. 98 shows this screen.

As shown in FIG. 98, in the screen according to the sixth embodiment, abinder 13 buries gaps among particles 5 in the particle layers 2 through4. It is important to use as the material of the binder 13 a substancehaving a refractive index different from that of the particles. Morespecifically, in case the particles are silica particles, for example,polyolefin-based materials such as polypropylene, polyethylene,polyisobutylene and polyvinyl acetate are usable as the binder 13.

For manufacturing this screen, there are some employable methods, suchas the method of first forming the particle layers 2 through 4 on thesubstrate 1, thereafter introducing a solution containing the bindermaterial into the particle layers 2 through 4 and curing it therein, andthe method of previously mixing a solution containing the bindermaterial into a colloid solution of particles (such as silica particles)such that the binder material fills gaps among the particles as theparticles accumulate.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the sixth embodiment, in addition to the same advantages asthose of the first embodiment, there are additional advantages, such asimprovement of the mechanical strength of the screen by the binder 13filling gaps among the particles 5 improve, improvement of the opticalproperty such as narrowing the full width at half maximum of thereflection spectrum by controlling the refractive index of the binder 13relative to the material of the particles 5.

Next explained is a screen according to the seventh embodiment of theinvention. FIG. 99 shows this screen.

As shown in FIG. 99, in the screen according to the seventh embodiment,the portions corresponding to the particles 5 in the particle layers 2through 4 of the screen shown in FIG. 98 remain as voids 14 to form aso-called inverse opal structure.

This screen can be manufactured by once forming the particle layers 2through 4 on the substrate 1, then introducing a binder material intothe particle layers 2 through 4, thereafter curing it therein to fillgaps among the particles 5, and thereafter immersing the substrate 1 andthe layers 2 to 4 into a predetermined etchant such as a hydrofluoricacid solution to dissolve the particles (such as silica particles).

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the seventh embodiment, in addition to the same advantagesas those of the first embodiment, there is additional advantage that alarger difference of refractive indices can be obtained than thatobtained when the particles 5 are silica particles, for example, becausethe difference of refractive indices between the voids 14 correspondingto particles and the binder 13 is the difference of refractive indicesbetween air and the binder 13. As a result, cycles of lamination of theparticle layers that are necessary for ensuring required reflection canbe reduced, and this contributes to reducing the thickness of thescreen.

Next explained is a screen according to the eighth embodiment of theinvention. FIG. 100 shows this screen.

As shown in FIG. 100, in the screen according to the eighth embodiment,a transparent substrate 15 having absorption layer 16 on its backsurface is used as the substrate. The absorption layer 16 may be made ofa material that can absorb light of wavelengths other than those ofthree primary colors. For example, a carbon film may be used. Morespecifically, the transparent substrate 15 may be, for example, atransparent glass substrate or polycarbonate substrate, and theabsorption layer 16 may be a carbon film coated on the back surface ofthe substrate.

Thickness of the absorption layer 16 is determined depending on itsmaterial so as to sufficiently absorb light of wavelengths other thanthree primary colors. Thickness of the absorption layer 16, when it is acarbon film, is explained here. That is, absorption coefficient α ofcarbon is generally 10³˜10⁵ cm⁻¹, although it depends on themanufacturing method when the traveling distance of light in theabsorption layer 16 is x, the intensity of light P is expressed byP(x)/P(0)=exp(−αx). Therefore, in case of α=10⁵ cm⁻¹, thickness d of thecarbon film may be 0.1 μm for weakening the intensity of light to 1/e(e: base of the natural logarithm) as sufficient absorption. Therefore,thickness at least of d=0.1 μm is necessary. Further, for weakening theintensity of light to 1/e also under α=10³ cm⁻¹, thickness of the carbonfilm must be d=10 μm. Taking account of them, it is important that thethickness of the carbon film is not smaller than 0.1 μm, and morepreferably, not less than 10 μm.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the eighth embodiment, in addition to the same advantagesas those of the first embodiment, there is the additional advantage thatthe material of the substrate can be selected more freely because thesubstrate itself may be incapable of absorbing of light.

Next explained is a screen according to the ninth embodiment of theinvention. FIG. 101 shows this screen.

As shown in FIG. 101, in the screen according to the ninth embodiment, ablack (capable of absorbing light of wavelengths other than threeprimary colors) PET film 17 having a rough surface made by surfaceroughening by sand processing is used as the substrate. Sand processingis a surface treatment for roughening the surface by rubbing it with afile, for example. Height from the bottom to the top of the irregularityon the surface of the PET film 17 may be, for example, 0.8˜4 μm. In thiscase, the roughed surface of the PET film 17 exhibits a goodwettability, a solution 6 containing particles 5 like silica particlesdiffused therein can be easily coated. Further, since the surfaceirregularity of the PET film 17 moderates the directivity of light, hotspots are unlikely to generate.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the ninth embodiment, in addition to the same advantages asthose of the first embodiment, there is the additional advantage thatthe use of the inexpensive PET film 17 as the substrate decreases themanufacturing cost of the screen.

Next explained is a screen, according to the tenth embodiment of theinvention. FIG. 102 shows this screen.

In the first embodiment already explained, the red-reflecting particlelayer 2, green-reflecting particle layer 3 and blue-reflecting particlelayer 4 are sequentially, vertically stacked on the substrate 1.However, the stacking order of the particle layer 2 to 4 need not alwaysfollow this order. From the viewpoint of better alignment (crystalproperty) of particles 5, the opposite stacking order is ratherpreferable. In the tenth embodiment, a structure stacking the particlelayers 2 to 4 in the opposite order will be explained.

That is, as shown in FIG. 102, in the screen according to the tenthembodiment, the blue-reflecting particle layer 4, green-reflectingparticle layer 3 and red-reflecting particle layer 2 are sequentiallystacked on the substrate 1. In this case, since the grain size of theparticles 5 in the blue-reflecting particle layer 4 is the smallest.Therefore, if the particles 5 are aligned on the substrate 1, thesurface irregularity of the particle layer is smallest. In case theparticles of the green-reflecting particle layer 3 having the nextlarger grain size are aligned on the base surface with the smallestirregularity, their alignment is unlikely to get disordered, the theircrystal quality is improved. Similarly, also when the particles 5 of thered-reflecting particle layer 2 having the next larger grain size arealigned on the particle layer 3, their alignment is unlikely to getdisordered, and the crystal quality is improved. In this manner, thecrystal property can be improved in all particle layers 2 to 4.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the tenth embodiment, in addition to the same advantages asthose of the first embodiment, there is the additional advantage that,because of the good crystal quality in all particle layers, the fullwidth at half maximum of the reflection spectrum is narrow, and it ispossible to efficiently reflect three primary colors while reliablyabsorbing the other part of light into the substrate 1.

Next explained is a screen according to the eleventh embodiment of theinvention. FIG. 103 shows this screen.

As shown in FIG. 103, in the screen according to the eleventhembodiment, the red-reflecting particle layer 2, green-reflectingparticle layer 3 and blue-reflecting particle layer 4 are sequentiallystacked on the substrate 1 via a buffer layer 18. The buffer layer 18 isa particle layer made up of particles that are smaller in diameter thanthe particles of the blue-reflecting particle layer 4, namely, particlesof D=120 nm, for example.

The screen is manufactured by first stacking the particle layer as thebuffer layer 18 on the substrate and thereafter stacking particle layers2 to 4 thereon.

In the other respects, the screen taken here is the same as the screenaccording to the first embodiment, so their detailed explanation isomitted.

According to the eleventh embodiment, in addition to the same advantagesas those of the first embodiment, there are the following additionaladvantages. Since the buffer layer 18 in for of the particle layer isfirst stacked on the substrate 1 and the particle layers 2 to 4 arestacked thereon, the wettability of the base layer of the particle layer2 to 4 is improved as compared with the structure directly stacking theparticle layers 2 to 4 on the substrate 1. As a result, the crystalquality of the particle layers 2 to 4 can be improved. Furthermore,since the diameter of particles of the particle layer as the bufferlayer 18 is D=120 nm that is smaller than that of the blue-reflectingparticle layer 4, when light is projected onto the screen, wavelength ofBragg's reflection from the buffer layer 18 is shorter than thewavelength of visible light, and the Bragg's reflection does notadversely affects the screen property.

Next explained is an image display system according to the twelfthembodiment of the invention. FIG. 104 shows configuration of this imagedisplay system, and FIG. 105 shows a perspective view of the same imagedisplay system.

As shown in FIGS. 104 and 105, the image display system according to thetwelfth embodiment comprises a screen 19 according to any of the firstto eleventh embodiment, and a projector 20 for projecting images ontothe screen 19. The projector 20 includes a light source 21 capable ofemitting red, green and blue light, and condenser and projector lenses22, 23. The light source 21 is comprises semiconductor light emittingelements, which may be either semiconductor lasers or light emittingdiodes, capable of emitting red, green and blue light. Morespecifically, in case of using semiconductor lasers as the light source21, for example, an AlGaInP compound semiconductor laser may be uses asthe red-emitting semiconductor laser, a ZnSe compound semiconductorlaser as the green-emitting semiconductor laser, and a GaN compoundsemiconductor laser as the blue-emitting semiconductor laser.

Heretofore, the invention has been explained by way of some embodiments.The invention, however, is not limited to these embodiments but involvesvarious changes or modifications within the technical concept and scopeof the invention.

For example, numerical values, structures, shapes, materials and methodof stacking particles shown and explained in conjunction with theembodiments are not but mere examples, and other numerical values,structures, shapes, materials and methods for stacking particles may beused alternatively.

Further, although the third embodiment has been explained as providingthe micro lens film 11 on the top surface of the particle layer 4 andthe fourth embodiment has been explained as providing the micro prismfilm 12 on the top surface of the particle film 4, it is also acceptableto provide a film made up of a mixture of micro lenses and micro prismson the top surface of the particle layer 4.

As described above, according to the invention, clean images can beobtained in which black-displaying portions are lowered in luminancelevel without deterioration of the contrast of the images even whenexternal light irrelevant to the images intrudes onto the screen. Inaddition, with the screen according to the invention, a dark room neednot always used for projection, but even upon projection under normalfluorescent lamps or under open air, the contrast does not degrade.

Furthermore, in case images are formed by projecting light fromsemiconductor lasers or LEDs, which is narrow in the full width at halfmaximum and excellent in color purity, by efficiently, selectivelyreflecting light of images exclusively and cutting light of otherwavelengths, it is possible to maintain a high contrast and tosignificantly lower the luminance level of black-displaying portions.Moreover, even when light having a wide full width at half maximum ofthe spectrum of each primary color is projected from a liquid crystalprojector, for example, the color reproducibility on the chromaticitydiagram is improved, and pure colors can be expressed.

1. An image display system comprising: a screen having a structure inwhich particles having a size not larger than 1 μm are regularlyaligned, said screen including a layer or a bulk substrate for absorbingvisible light; and a projector light source including semiconductorlight emitting devices each for emitting light of a specific wavelengthdetermined by the size and alignment of said particles; wherein saidlayer or bulk substrate for absorbing visible light absorbs visiblelight of all wavelength bands.
 2. An image display system comprising: ascreen configured to reflect light of specific wavelengths by usingphotonic crystals, said screen including a layer or a bulk substrate forabsorbing visible light; and a projector light source includingsemiconductor light emitting devices for emitting light of said specificwavelengths; wherein said layer or bulk substrate for absorbing visiblelight absorbs visible light of all wavelength bands.
 3. An image displaysystem comprising: a screen configured to reflect light of specificwavelengths by using a dielectric multi-layered film, said screenincluding a layer or a bulk substrate for absorbing visible light; and aprojector light source including semiconductor light emitting devicesfor emitting light of said specific wavelengths; wherein said layer orbulk substrate for absorbing visible light absorbs visible light of allwavelength bands.
 4. A screen comprising particles regularly aligned toreflect electromagnetic waves of specific wavelengths, said screenincluding a layer or a bulk substrate for absorbing visible light;wherein said layer or bulk substrate for absorbing visible light absorbsvisible light of all wavelength bands.
 5. The screen according to claim4 wherein said electromagnetic waves are visible light.
 6. A screencomprising: first particles regularly aligned to reflect anelectromagnetic wave of a first wavelength; and second particlesregularly aligned to reflect an electromagnetic wave of a secondwavelength different from said first wavelength, wherein said firstparticles and said second particles are different in diameter; whereinsaid screen includes a layer or a bulk substrate for absorbing visiblelight; and wherein said layer or bulk substrate for absorbing visiblelight absorbs visible light of all wavelength bands.
 7. The screenaccording to claim 6 wherein both said electromagnetic wave of saidfirst wavelength and said electromagnetic wave of said second wavelengthare visible light.